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The National Republican Senatorial Committee launched its first TV ad of the cycle in Kentucky on Tuesday to help Rand Paul, who ousted establishment favorite Trey Grayson in the primary. Paul is fighting a close race against Democrat Jack Conway.

Florida is one of the biggest unknowns, as Gov. Charlie Crist left the GOP to run as an Independent against Republican Marco Rubio and Democratic Rep. Kendrick Meek.

Ohio and New Hampshire are also open GOP seats, but neither is viewed as as competitive as the other three.

Alaska is another race where Democrats might look to make things interesting. But that all depends on whether Sen. Lisa Murkowski, defeated in last months GOP primary, decides to run as a write-in candidate.

A GOP operative noted that in January 2009, when there had been a string of Republican retirements in competitive states, it looked like Democrats could increase their majority even more. But a year later, the landscape had changed dramatically and the Jan. 19 special-election win by Sen. Scott Brown (R-Mass.) brought home the reality of the political environment for Democrats.

Privately, Democrats say they do not see losing the majority as a likely scenario but concede anything is possible given the turbulent cycle and angry mood of many voters.

DSCC Chairman Bob Menendez said Republican primary miscues  including in Delaware, Florida, Kentucky and Nevada, among others  have helped keep a difficult environment for Democrats from getting worse.

Republicans have chosen extremists to be their nominees, and this has changed the political map for the cycle, the New Jersey Democrat said. We are demonstrably more competitive in a handful of states because they have selected to run extremists instead of mainstream candidates. These are candidates who are much more focused on adopting a strict social doctrine than the economic challenges facing working people.

NRSC Chairman John Cornyn (Texas) has said that winning a majority is a more likely prospect in 2012. Still, Republicans have put enough seats in play to do it this year.

Thats thanks to an expansion of the playing field that has allowed the GOP to compete in states that havent elected a Republican to the Senate in years: California (1988), Connecticut (1982), Washington (1994) and Wisconsin (1986).

Connecticut is seen as the longest shot among those four states, and a loss there on election night would probably mean Democrats hold on the majority is in dire shape, operatives said. Polling has shown the race tightening over the past few months; a Quinnipiac University poll released Tuesday found Blumenthal, the state attorney general, leading McMahon, the former CEO of World Wrestling Entertainment, 51 percent to 45 percent.